Evaluation of phase equilibria in the Nb-rich portion of Nb-B system

TitleEvaluation of phase equilibria in the Nb-rich portion of Nb-B system
Publication TypeJournal Article
Year of Publication2008
AuthorsTang ZH, Kramer MJ, Akinc M
Journal TitleIntermetallics
Volume16
Pages255-261
Date PublishedFeb
Type of ArticleArticle
ISBN Number0966-9795
Accession NumberISI:000253275200017
Keywordsintermetallics, microstructure, miscellaneous, phase diagrams, REGION, thermal properties
Abstract

The phase equilibria in the Nb-rich portion of Nb-B system have been evaluated experimentally using metallographic analysis, differential thermal analysis (DTA) and X-ray diffraction. It showed that Nb-ss (solid solution) and NbB are the only two primary phases in the 0-40 at.% B composition range, and the eutectic reaction L <-> Nb-ss + NbB exists, instead of the generally accepted reaction L <-> Nb-ss + Nb3B2, as indicated in the Nb-B phase diagram. The Nb3B2 phase, however, forms by the peritectoid reaction Nb-ss + NbB <-> Nb3B2. DTA tests were conducted on annealed Nb-14B, Nb-16B, Nb-18B and Nb-40B alloys, and temperature and heat of phase transition were determined. The eutectic reaction (L <-> Nb-ss + NbB) temperature was determined to be 2104 +/- 5 degrees C, and the heat of phase transition was estimated as 22-30 kJ/mol, depending on the method of calibration used. The thermal event associated with peritectoid reactions was not observed in DTA curves due to sluggish solid state transformation, but the thermal annealing experiments show that peritectoid temperature is above 1900 degrees C. (C) 2007 Elsevier Ltd. All rights reserved.

DOI10.1016/j.intermet.2007.10.004
Alternate JournalIntermetallics